Opto-electronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Examples of organic opto-electronic devices include organic light emitting devices (OLEDs), organic transistors/phototransistors, organic photovoltaic cells, and organic photodetectors. For OLEDs, the organic materials may have performance advantages over conventional (i.e., inorganic) materials. For example, the wavelength at which an organic emissive layer emits light may generally be readily tuned with appropriate dopants. For organic transistors/phototransistors, the substrates upon which they are constructed may be flexible, providing for broader applications in industry and commerce.
As used herein, the term “organic” includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic devices including opto-electronic devices. “Small molecule” refers to any organic material that is not a polymer, and “small molecules” may actually be quite large. Small molecules may include repeat units in some circumstances. For example, using a long chain alkyl group as a substituent does not remove a molecule from the “small molecule” class. Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone. Small molecules may also serve as the core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety. Small molecules generally have a well defined molecular weight, whereas polymers generally do not have a well defined molecular weight.
General background information on small molecular weight organic thin-film photodetectors and solar cells may be found in Peumans et al., “Small Molecular Weight Organic Thin-Film Photodetectors and Solar Cells,” Journal of Applied Physics-Applied Physics Reviews-Focused Review, Vol. 93, No. 7, pp. 3693-3723 (April 2003).
The “fill factor” (FF) of a solar cell is Pmax/(Jsc*Voc), where Pmax is the maximum power of the solar cell, determined by finding the point on the I-V curve for which the product of the current and voltage is a maximum. A high FF is an indication of how “square” the I-V curve for a solar cell appears.
Optoelectronic devices rely on the optical and electronic properties of materials to either produce or detect electromagnetic radiation electronically or to generate electricity from ambient electromagnetic radiation. Photosensitive optoelectronic devices convert electromagnetic radiation into electricity. Photovoltaic (PV) devices or solar cells, which are a type of photosensitive optoelectronic device, are specifically used to generate electrical power. PV devices, which may generate electrical power from light sources other than sunlight, are used to drive power consuming loads to provide, for example, lighting, heating, or to operate electronic equipment such as computers or remote monitoring or communications equipment. These power generation applications also often involve the charging of batteries or other energy storage devices so that equipment operation may continue when direct illumination from the sun or other ambient light sources is not available. As used herein the term “resistive load” refers to any power consuming or storing device, equipment, or system. Another type of photosensitive optoelectronic device is a photoconductor cell. In this function, signal detection circuitry monitors the resistance of the device to detect changes due to the absorption of light. Another type of photosensitive optoelectronic device is a photodetector. In operation a photodetector has a voltage applied and a current detecting circuit measures the current generated when the photodetector is exposed to electromagnetic radiation. A detecting circuit as described herein is capable of providing a bias voltage to a photodetector and measuring the electronic response of the photodetector to ambient electromagnetic radiation. These three classes of photosensitive optoelectronic devices may be characterized according to whether a rectifying junction as defined below is present and also according to whether the device is operated with an external applied voltage, also known as a bias or bias voltage. A photoconductor cell does not have a rectifying junction and is normally operated with a bias. A PV device has at least one rectifying junction and is operated with no bias. A photodetector has at least one rectifying junction and is usually but not always operated with a bias.
A need exists for an organic photovoltaic cells with a higher efficiency.